This invention relates to methods and apparatus for testing the compressive strengths of cement and cement slurry mixtures. More particularly the invention relates to non-destructive methods and apparatus for testing such cement slurries and cement samples.
Historically, in order to determine the compressive strength of cement as a function of time it has been necessary to prepare a multiplicity of samples of the cement in small test cylinders, or cubes, and to then destructively test (or crush) a series of such samples as a function of time as the cement cured. In testing batches of cement in this manner, while the cement may be cured at elevated temperatures and pressures such as are present in a wellbore environment, it has been necessary to remove the cement samples from the elevated temperature and pressure environment in order to perform the crushing test on the strength measuring machines required for this purpose. Thus the actual testing of cement samples has been performed usually at room temperature and at atmospheric pressure, rather than at the actual temperature and pressure present in a well borehole environment.
It is apparent that such prior art methods for testing the compressive strengths of cements as a function of time have many shortcomings. For example, the samples taken from a particular batch of cement to form the multiplicity of cylinders required for such testing may not be uniform. Similarly, the removal of the sample cylinders from the elevated temperature and pressure conditions for testing at atmospheric pressure and room temperature could conceivably alter the physical properties of the cement, i.e., the very properties being measured. Moreover, such testing is inconvenient, cumbersome and time consuming.
The time required for oil or gas well cementing slurry to reach an initial set and to develop useful compressive strength are important parameters in scheduling further drilling or completion operations. For example, operations such as well temperature logs to locate the top of a cement column, cement bond logs to evaluate cement jobs, the continuation of drilling after setting intermediate casing or liners, the perforating of production zones, or the clean out and well stimulation treatments after such operations as squeeze cementing, all require a knowledge of the compressive strength and initial set time of the cement slurry used in the operation.
Without reliable set time and compressive strength data, temperature logs and cement bond logs can be run to soon or delayed unnecessarily. Additional drilling in a well borehole can also be carried out either prematurely or later than needed. If well clean up and stimulation treatments are carried out too soon after a squeeze cement treatment, interzone communication problems may develop. Similarly, it has been found that well perforating during the early stages of strength development of the cement slurry around a well casing can result in cleaner perforations with less shattering and giving better zone isolation. From these examples, it may be seen that a precise knowledge of the initial set time and strength development of the cement slurry used in well completions or treatments are essential to perform the above type operations at the optimum time to avoid unnecessary delays.